Inspection Apparatus and Method

ABSTRACT

A spectroscopic scatterometer detects both zero order and higher order radiation diffracted from an illuminated spot on a target grating. The apparatus forms and detects a spectrum of zero order (reflected) radiation, and separately forms and detects a spectrum of the higher order diffracted radiation. Each spectrum is formed using a symmetrical phase grating, so as to form and detect a symmetrical pair of spectra. The pair of spectra can be averaged to obtain a single spectrum with reduced focus sensitivity. Comparing the two spectra can yield information for improving height measurements in a subsequent lithographic step. The target grating is oriented obliquely so that the zero order and higher order radiation emanate from the spot in different planes. Two scatterometers can operate simultaneously, illuminating the target from different oblique directions. A radial transmission filter reduces sidelobes in the spot and reduces product crosstalk.

FIELD

The present invention relates to apparatus and methods of inspectionusable, for example, in the manufacture of devices by lithographictechniques.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.,comprising part of, one, or several dies) on a substrate (e.g., asilicon wafer). Transfer of the pattern is typically via imaging onto alayer of radiation-sensitive material (resist) provided on thesubstrate. In general, a single substrate will contain a network ofadjacent target portions that are successively patterned. Knownlithographic apparatus include so-called steppers, in which each targetportion is irradiated by exposing an entire pattern onto the targetportion at one time, and so-called scanners, in which each targetportion is irradiated by scanning the pattern through a radiation beamin a given direction (the “scanning”-direction) while synchronouslyscanning the substrate parallel or anti-parallel to this direction. Itis also possible to transfer the pattern from the patterning device tothe substrate by imprinting the pattern onto the substrate.

In order to monitor the lithographic process, parameters of thepatterned substrate are measured. Parameters may include, for example,the overlay error between successive layers formed in or on thepatterned substrate and critical linewidth of developed photosensitiveresist. This measurement may be performed on a product substrate and/oron a dedicated metrology target. There are various techniques for makingmeasurements of the microscopic structures formed in lithographicprocesses, including the use of scanning electron microscopes andvarious specialized tools. A fast and non-invasive form of specializedinspection tool is a scatterometer in which a beam of radiation isdirected onto a target on the surface of the substrate and properties ofthe scattered or reflected beam are measured. By comparing theproperties of the beam before and after it has been reflected orscattered by the substrate, the properties of the substrate can bedetermined. This can be done, for example, by comparing the reflectedbeam with data stored in a library of known measurements associated withknown substrate properties. Two main types of scatterometer are known.Spectroscopic scatterometers direct a broadband radiation beam onto thesubstrate and measure the spectrum (intensity as a function ofwavelength) of the radiation scattered into a particular narrow angularrange. Angularly resolved scatterometers use a monochromatic radiationbeam and measure the intensity of the scattered radiation as a functionof angle.

Spectroscopic scatterometry has the benefit of the hardware isrelatively simple, which helps to improve matching and calibration.However, it has difficulties measuring very isolated features, andasymmetry of the metrology gratings. Angle-resolved scatterometry isoptically more complicated, which complicates calibration and matching.Moreover, in practice, multiple adjustable wavelengths are needed, whichleads to complex and expensive optics. As the resolution of lithographicprocesses increases, ever smaller features are created on substrates. Inorder to perform scatterometry at the resolution of the smallestfeatures, it may be desirable to use shorter wavelengths of radiation,comparable to those used in the lithographic process itself. Wavelengthsin the ultraviolet (UV) range may be effective for this in principle.However, optical systems for such wavelengths become particularlycomplex.

There is accordingly a desire for new forms of scatterometer,particularly ones suitable for measuring metrology targets with featuresizes at the resolution of current and next-generation lithographicprocesses. The inventor has recognized that a limitation of knownspectroscopic scatterometers is that they make no use of higherdiffracted radiation from the target grating.

A new form of scatterometer has been proposed in the paper “A NewApproach to Pattern Metrology” Christopher P. Ausschnitt, published inMetrology, Inspection, and Process Control for Microlithography XVIII,edited by Richard M. Silver, Proceedings of SPIE Vol. 5375 (SPIE,Bellingham, Wash., 2004), DOI: 10.1117/12.539143. Unlike conventionalspectroscopic scatterometers, Ausschnitt's so-called MOXIE system usesboth zero order and first order diffracted radiation. It also uses atarget grating on the substrate itself to resolve the diffracted ordersinto a spectroscopic signal. However, this system is also not optimizedfor measuring pattern asymmetries. Moreover, the spectral resolution ofthe first order signal is dependent on the target geometry and isexpected to be too small for practical metrology applications.

Another problem in known scatterometry techniques is the space or “realestate” occupied by scatterometry targets on product substrates. Targetsmust be kept away from one another and from product features, to avoidcross-talk between measurements. The inventor has further recognizedthat one cause of cross-talk is that an illumination spot of theinstrument has a point spread function with significant sidelobes ofenergy around a main spot.

A problem in lithographic processes generally is that heightmeasurements used for controlling the transfer of a pattern to asubstrate may be influenced unpredictably by process-dependentinfluences.

SUMMARY

According to first aspect of the present invention, there is providedinspection apparatus comprising a spectroscopic scatterometer havingillumination optics for directing broadband radiation with an angle ofincidence at a spot on a target structure, the target structure in usecomprising a periodic grating, zero order detection optics for receivingradiation reflected from the target and for forming and detecting aspectrum of the reflected radiation, and higher order detection opticsfor receiving radiation diffracted at one or more higher orders by theperiodic grating in the target structure, and for forming and detectinga spectrum of the received diffracted radiation.

In one embodiment, the illumination optics and zero order detectionoptics process radiation generally in a first plane perpendicular to aplane the target structure, while the higher order detection optics arearranged to process radiation in a second plane, angled relative to thefirst plane, the higher order diffracted radiation entering the secondplane during use of the apparatus as a result of an oblique orientationof lines the periodic grating in the target structure, relative to thefirst plane.

Putting the higher order radiation into a different plane facilitatesthe practical layout of such an instrument. In one embodiment the firstplane is set at an angle of 45° relative to the expected orientation ofthe lines in the periodic grating, and a second spectroscopicscatterometer is provided for simultaneous measurement of the sametarget structure, a first plane of the second spectroscopicscatterometer being arranged at right angles to the first plane of thefirst-mentioned spectroscopic scatterometer. Optical components can beshared between the higher order detection optics of the first-mentionedand second scatterometers.

The present invention in the first aspect further provides a method ofmeasuring properties of a target structure on a substrate, a targetstructure including a periodic grating, the method comprising obtaininga zero order spectrum and at least one higher order spectrum from thetarget structure using an inspection apparatus according to the firstaspect of the present invention as set forth above, and processing themeasured spectra so as to obtain measurements of one or more parametersof the target structure.

According to a second, independent aspect of the present invention,there is provided a spectroscopic scatterometer having illuminationoptics for directing broadband radiation with an angle of incidence at aspot on a target structure, detection optics for receiving radiationdiffracted at a zero or higher order from the target structure and forforming and detecting a spectrum of the diffracted radiation, whereinthe detection optics comprises a symmetric diffraction grating arrangedto form a symmetrical pair of spectra of the reflected radiation, andwherein a pair of spectrum detectors are arranged to detect both of thespectra, the apparatus further comprising a processor for combiningmeasurements from both detectors to obtain a single detected spectrum ofthe reflected radiation.

A spectroscopic scatterometer according to the second aspect of thepresent invention can have a very simple optical construction comparedwith known spectroscopic apparatus, suitable for example to use UVradiation.

A spectroscopic scatterometer according to the second aspect of thepresent invention with a symmetrical pair of spectra can be made to havereduced sensitivity to defocus, compared with a conventionalsingle-spectrum arrangement. In one embodiment, a further detector isarranged to receive a zero order beam of the diffraction grating, thefurther detector being located at a point generally in between the pairof spectrum detectors, and signals from the further detector are usedfor monitoring focus of the spot on the target structure.

A spectroscopic scatterometer according to the second aspect of thepresent invention may be used to form the zero order and/or the higherorder detection optics in an apparatus according to the first aspect ofthe present invention. Examples further below will illustrate this withreference to the drawings.

The present invention in the second aspect further provides a method ofmeasuring properties of a target structure on a substrate, a targetstructure including a periodic grating, the method comprising obtaininga spectrum from the target structure using an inspection apparatusaccording to the second aspect of the present invention as set forthabove, and processing the detected spectrum so as to obtain measurementsof one or more parameters of the target structure.

The invention in a third aspect provides an optical apparatus includingillumination optics for focusing a beam of radiation at a spot on atarget structure, wherein a filter is provided for imposing on the beam(prior to focusing) a transmission loss increasing with radial distancefrom an optical axis of the beam, thereby reducing energy in sidelobesin a point spread function of the illumination optics.

The optical apparatus including the illumination optics may be ascatterometer according to the first or second aspect of the inventionfor example. The suppression of sidelobes in the point spread functioncan reduce cross-talk between neighboring features, for example to allowmore accurate measurements of target properties, and/or the use ofsmaller targets closer to neighboring structures.

The invention in the third aspect further provides a method of measuringproperties of a target structure on a substrate, the method comprisingilluminating the target structure with a spot of radiation using anoptical apparatus according to the third aspect of the invention as setforth above, detecting radiation diffracted by the target structure andprocessing the detected radiation to obtain measurements of one o moreparameters of the target structure.

The inventor has recognized that a spectroscopic scatterometer havingthe symmetrical pair of spectra (similar to that used the second aspectof the invention described above) can alternatively or in addition beused to obtain information of process-dependent variations influencingheight measurements made for control of a lithographic patterningprocess. These influences, which may vary from substrate-to-substrateand lot-to-lot, may cause a height sensor to report a different heightthan the true surface height. This height error may be referred to as anapparent surface depression (ASD), and causes inaccuracy in a subsequentlithographic step that uses height measurements for example to focus anoptical projection system.

According to a fourth, independent aspect of the present invention,there is provided a spectroscopic scatterometer having illuminationoptics for directing broadband radiation with an angle of incidence at aspot on a target structure, detection optics for receiving radiationdiffracted at a zero or higher order from the target structure and forforming and detecting a spectrum of the diffracted radiation, whereinthe detection optics comprises a symmetric diffraction grating arrangedto form a symmetrical pair of spectra of the reflected radiation, andwherein a pair of spectrum detectors are arranged to detect both of thespectra, the apparatus further comprising a processor for comparingmeasurements from both detectors to obtain information of an apparentsurface depression at the location of the target structure on asubstrate.

The invention in the fourth aspect further provides a method ofcontrolling a lithographic apparatus wherein a pattern is applied to asubstrate based on height measurements made by the apparatus at one ormore locations across a substrate, and wherein a correction is appliedto the height measurements based on information of an apparent surfacedepression obtained using a spectroscopic scatterometer according to thethird aspect of the invention, set forth above.

The various aspects of the invention set forth above can be usedindependently of one another, or two or more aspects may be combinedtogether in the same apparatus or method.

Further features and advantages of the present invention, as well as thestructure and operation of various embodiments of the present invention,are described in detail below with reference to the accompanyingdrawings. It is noted that the present invention is not limited to thespecific embodiments described herein. Such embodiments are presentedherein for illustrative purposes only. Additional embodiments will beapparent to persons skilled in the relevant art(s) based on theteachings contained herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and form partof the specification, illustrate exemplary embodiments of the presentinvention and, together with the description, further serve to explainthe principles of the present invention and to enable a person skilledin the relevant art(s) to make and use the present invention.

FIG. 1 depicts a lithographic apparatus.

FIG. 2 depicts a lithographic cell or cluster.

FIG. 3 is a schematic block diagram of a novel inspection apparatusembodying a first aspect of the present invention.

FIG. 4 is a schematic block diagram of a zero order branch in theapparatus of

FIG. 3, embodying a second aspect of the present invention, with insetdetail of a Rochon prism polarizer.

FIG. 5 is a schematic diagram of the zero order branch in a practicalembodiment of the apparatus of FIG. 3.

FIGS. 6( a) and 6(b) illustrate features of a spectroscopic grating inthe apparatus of FIGS. 3 to 5.

FIG. 6( c) illustrates the displacement of spectra of differentlypolarized radiation generated by the grating in the apparatus.

FIGS. 7( a), 7(b) and 7(c) illustrate the management of a photon budgetin an example apparatus.

FIGS. 8 and 9 illustrate the management of aberration in a spectrumdetected in the apparatus of FIGS. 3 to 5.

FIG. 10 illustrates the management of focus error in the apparatus ofFIGS. 4 and 5.

FIG. 11 illustrates a novel calibration prism useful in setting up theapparatus of FIGS. 3 to 5.

FIG. 12 illustrates the layout of a dual-branch scatterometer includingtwo of the apparatus of FIGS. 3 to 5 in an embodiment with obliqueorientation relative to a target grating direction, and showing thegeneral location of first order diffracted radiation in a differentplane to the zero order radiation.

FIGS. 13( a) to 13(d) illustrate the collection of higher orderdiffracted radiation in the dual-branch scatterometer shown in FIG. 12.

FIG. 14 shows in more detail the form of a filter in the scatterometerfor mitigating product cross-talk in the scatterometers of FIGS. 3 to12, in accordance with the third aspect of the invention.

FIGS. 15( a) to 15(d) comprise graphs illustrating various parameters ofthe filter of FIG. 14 and its effect on a point spread function of thescatterometer.

FIGS. 16( a) to 16(c) illustrate the phenomenon of apparent surfacedepression affecting height measurements used for example in alithographic process.

FIG. 17 illustrates a method of obtaining information on apparentsurface depression using a scatterometer of the type illustrated inFIGS. 4 and 5, in accordance with the fourth aspect of the invention.

FIG. 18 is a flowchart of a method of patterning substrates according toan embodiment of the invention in its fourth aspect.

The features and advantages of the present invention will become moreapparent from the detailed description set forth below when taken inconjunction with the drawings, in which like reference charactersidentify corresponding elements throughout. In the drawings, likereference numbers generally indicate identical, functionally similar,and/or structurally similar elements.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

This specification discloses one or more embodiments that incorporatethe features of this present invention. The disclosed embodiment(s)merely exemplify the present invention. The scope of the presentinvention is not limited to the disclosed embodiment(s). The presentinvention is defined by the claims appended hereto.

The embodiment(s) described, and references in the specification to “oneembodiment”, “an embodiment”, “an example embodiment”, etc., indicatethat the embodiment(s) described may include a particular feature,structure, or characteristic, but every embodiment may not necessarilyinclude the particular feature, structure, or characteristic. Moreover,such phrases are not necessarily referring to the same embodiment.Further, when a particular feature, structure, or characteristic isdescribed in connection with an embodiment, it is understood that it iswithin the knowledge of one skilled in the art to effect such feature,structure, or characteristic in connection with other embodimentswhether or not explicitly described.

Embodiments of the present invention may be implemented in hardware,firmware, software, or any combination thereof. Embodiments of thepresent invention may also be implemented as instructions stored on amachine-readable medium, which may be read and executed by one or moreprocessors. A machine-readable medium may include any mechanism forstoring or transmitting information in a form readable by a machine(e.g., a computing device). For example, a machine-readable medium mayinclude read only memory (ROM); random access memory (RAM); magneticdisk storage media; optical storage media; flash memory devices;electrical, optical, acoustical or other forms of propagated signals(e.g., carrier waves, infrared signals, digital signals, etc.), andothers. Further, firmware, software, routines, instructions may bedescribed herein as performing certain actions. However, it should beappreciated that such descriptions are merely for convenience and thatsuch actions in fact result from computing devices, processors,controllers, or other devices executing the firmware, software,routines, instructions, etc.

Before describing such embodiments in more detail, however, it isinstructive to present an example environment in which embodiments ofthe present invention may be implemented.

FIG. 1 schematically shows a lithographic apparatus LAP including asource collector module SO according to an embodiment of the presentinvention. The apparatus comprises: an illumination system (illuminator)IL configured to condition a radiation beam B (e.g., EUV radiation); asupport structure (e.g., a mask table) MT constructed to support apatterning device (e.g., a mask or a reticle) MA and connected to afirst positioner PM configured to accurately position the patterningdevice; a substrate table (e.g., a wafer table) WT constructed to hold asubstrate (e.g., a resist-coated wafer) W and connected to a secondpositioner PW configured to accurately position the substrate; and aprojection system (e.g., a reflective projection system) PS configuredto project a pattern imparted to the radiation beam B by patterningdevice MA onto a target portion C (e.g., comprising one or more dies) ofthe substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure supports, i.e., bears the weight of, thepatterning device. It holds the patterning device in a manner thatdepends on the orientation of the patterning device, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The support structure can use mechanical, vacuum, electrostatic or otherclamping techniques to hold the patterning device. The support structuremay be a frame or a table, for example, which may be fixed or movable asrequired. The support structure may ensure that the patterning device isat a desired position, for example with respect to the projectionsystem. Any use of the terms “reticle” or “mask” herein may beconsidered synonymous with the more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam, which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.,employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g., employing a programmable mirror array of a typeas referred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g., water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located between the projection systemand the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as a-outer anda-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam B is incident on the patterning device (e.g., maskMA), which is held on the support structure (e.g., mask table MT), andis patterned by the patterning device. Having traversed the mask MA, theradiation beam B passes through the projection system PL, which focusesthe beam onto a target portion C of the substrate W. With the aid of thesecond positioner PW and position sensor IF (e.g., an interferometricdevice, linear encoder, 2-D encoder or capacitive sensor), the substratetable WT can be moved accurately, e.g., so as to position differenttarget portions C in the path of the radiation beam B. Similarly, thefirst positioner PM and another position sensor (which is not explicitlydepicted in FIG. 1) can be used to accurately position the mask MA withrespect to the path of the radiation beam B, e.g., after mechanicalretrieval from a mask library, or during a scan. In general, movement ofthe mask table MT may be realized with the aid of a long-stroke module(coarse positioning) and a short-stroke module (fine positioning), whichform part of the first positioner PM. Similarly, movement of thesubstrate table WT may be realized using a long-stroke module and ashort-stroke module, which form part of the second positioner PW. In thecase of a stepper (as opposed to a scanner) the mask table MT may beconnected to a short-stroke actuator only, or may be fixed. Mask MA andsubstrate W may be aligned using mask alignment marks MK1, MK2 andsubstrate alignment marks P1, P2. Although the substrate alignment marksas illustrated occupy dedicated target portions, they may be located inspaces between target portions (these are known as scribe-lane alignmentmarks). Similarly, in situations in which more than one die is providedon the mask MA, the mask alignment marks may be located between thedies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the mask table MT and the substrate table WT are keptessentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e., asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the mask table MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e., a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the masktable MT may be determined by the (de-)magnification and image reversalcharacteristics of the projection system PL. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the mask table MT is kept essentially stationaryholding a programmable patterning device, and the substrate table WT ismoved or scanned while a pattern imparted to the radiation beam isprojected onto a target portion C. In this mode, generally a pulsedradiation source is employed and the programmable patterning device isupdated as required after each movement of the substrate table WT or inbetween successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

As shown in FIG. 2, the lithographic apparatus LA forms part of alithographic cell LC, also sometimes referred to a lithocell or cluster,which also includes apparatus to perform pre- and post-exposureprocesses on a substrate. Conventionally these include spin coaters SCto deposit resist layers, developers DE to develop exposed resist, chillplates CH and bake plates BK. A substrate handler, or robot, RO picks upsubstrates from input/output ports I/O1, I/O2, moves them between thedifferent process apparatus and delivers then to the loading bay LB ofthe lithographic apparatus. These devices, which are often collectivelyreferred to as the track, are under the control of a track control unitTCU which is itself controlled by the supervisory control system SCS,which also controls the lithographic apparatus via lithography controlunit LACU. Thus, the different apparatus can be operated to maximizethroughput and processing efficiency.

In order that the substrates that are exposed by the lithographicapparatus are exposed correctly and consistently, it is desirable toinspect exposed substrates to measure properties such as overlay errorsbetween subsequent layers, line thicknesses, critical dimensions (CD),etc. If errors are detected, adjustments may be made to exposures ofsubsequent substrates, especially if the inspection can be done soon andfast enough that other substrates of the same batch are still to beexposed. Also, already exposed substrates may be stripped andreworked—to improve yield—or discarded, thereby avoiding performingexposures on substrates that are known to be faulty. In a case whereonly some target portions of a substrate are faulty, further exposurescan be performed only on those target portions which are good.

An inspection apparatus is used to determine the properties of thesubstrates, and in particular, how the properties of differentsubstrates or different layers of the same substrate vary from layer tolayer. The inspection apparatus may be integrated into the lithographicapparatus LA or the lithocell LC or may be a stand-alone device. Toenable most rapid measurements, it is desirable that the inspectionapparatus measure properties in the exposed resist layer immediatelyafter the exposure. However, the latent image in the resist has a verylow contrast—there is only a very small difference in refractive indexbetween the parts of the resist which have been exposed to radiation andthose which have not—and not all inspection apparatus have sufficientsensitivity to make useful measurements of the latent image. Thereforemeasurements may be taken after the post-exposure bake step (PEB) whichis customarily the first step carried out on exposed substrates andincreases the contrast between exposed and unexposed parts of theresist. At this stage, the image in the resist may be referred to assemi-latent. It is also possible to make measurements of the developedresist image—at which point either the exposed or unexposed parts of theresist have been removed—or after a pattern transfer step such asetching. The latter possibility limits the possibilities for rework offaulty substrates but may still provide useful information.

FIG. 3 depicts a novel spectroscopic scatterometer 100 which processesboth zero order and higher order radiation, diffracted by a target 102on substrate W. The apparatus comprises a source 104 of broadbandradiation, illumination optics 106 zero order detection optics 108 anddetector 110. The scatterometer further comprises first order detectionoptics 120 and detector 122.

Such a scatterometer may be configured as a normal-incidencescatterometer or an oblique-incidence scatterometer. In the layoutillustrated in FIG. 3 and the subsequent diagrams, oblique incidence isused. A particular advantage of such a layout is that reflective opticsmay be used for the illumination optics 106 and the detection optics108, 120, simplifying the optical design when the radiation from source104 is in the deep UV (DUV) and vacuum UV (VUV) waveband. Thus a narrowbeam of radiation 140 received from source 104 is formed into a narrow,focused beam 142 to impinge on target 102. Zero order or specularreflected radiation 144 is received by zero order detection optics 108and split into a spectrum 146. First order radiation 150, diffracted byperiodic features within the target 102, is received by first orderdetection optics 120 and split into a spectrum 152. Spectra of zeroorder and higher order diffracted radiation are captured by thedetectors 110 and 122 respectively, and supplied to processing unit 130.Each detected spectrum records intensity I as a function of wavelengthλ. The zero order spectrum is represented by a function I⁰(λ), while afirst order spectrum is represented by a function I¹(λ). In thisillustration, only the first order radiation is captured as an exampleof a higher order. In other embodiments, second, third etc orders mayalso be captured. In the following description, the first orderdiffracted radiation will be referred to for simplicity, on theunderstanding that this is merely one example of higher order diffractedradiation.

From the data received from detectors 110 and 122, the structure orprofile giving rise to the detected spectrum may be reconstructed byprocessing unit 130. The target 102 on substrate W may be a 1-D grating,which is printed such that after development, the bars are formed ofsolid resist lines. The target 102 may be a 2-D grating, which isprinted such that after development, the grating is formed of solidresist pillars or vias in the resist. The bars, pillars or vias mayalternatively be etched into the substrate. This pattern is sensitive tochromatic aberrations in the lithographic projection apparatus,particularly the projection system PL, and illumination symmetry and thepresence of such aberrations will manifest themselves in a variation inthe printed grating. Accordingly, the scatterometry data of the printedgratings is used to reconstruct the gratings. The parameters of the 1-Dgrating, such as line widths and shapes, or parameters of the 2-Dgrating, such as pillar or via widths or lengths or shapes, may be inputto the reconstruction process, performed by processing unit 130, fromknowledge of the printing step and/or other scatterometry processes.

As described above, the target is on the surface of the substrate. Thistarget will often take the shape of a series of lines in a grating orsubstantially rectangular structures in a 2-D array. The purpose ofrigorous optical diffraction theories in metrology is effectively thecalculation of a diffraction spectrum that is reflected from the target.In other words, target shape information is obtained for CD (criticaldimension) uniformity and overlay metrology. Overlay metrology is ameasuring system in which the overlay of two targets is measured inorder to determine whether two layers on a substrate are aligned or not.CD uniformity is simply a measurement of the uniformity of variousgratings across the substrate to determine how the exposure system ofthe lithographic apparatus is functioning. Specifically, CD, or criticaldimension, is the width of the object that is “written” on the substrateand is the limit at which a lithographic apparatus is physically able towrite on a substrate.

Using the scatterometers described above in combination with modeling ofa target such as the target 102 and its diffraction properties,measurement of the shape and other parameters of the structure can beperformed in a number of ways. In a first type of process (model-basedreconstruction), a diffraction pattern based on a first estimate of thetarget shape (a first candidate structure) is calculated and comparedwith the observed diffraction pattern. Parameters of the model are thenvaried systematically and the diffraction re-calculated in a series ofiterations, to generate new candidate structures and so arrive at a bestfit. In a second type of process, diffraction spectra for many differentcandidate structures are calculated in advance to create a ‘library’ ofdiffraction spectra. Then the diffraction pattern observed from themeasurement target is compared with the library of calculated spectra tofind a best fit. Both of these reconstruction methods are in principleknown to the skilled reader from using other types of scatterometer. Theskilled reader will be able readily to adapt the reconstruction methodsto use spectra obtained by the novel instrument disclosed herein. Bothmodel-based and library-based methods can be used together. For example,a coarse fit can be obtained from a library, followed by an iterativeprocess to find a best fit. Examples of techniques for performingmodel-based reconstruction are Rigorous Coupled Wave Analysis andnon-linear regression. In general, for the reconstruction the generalform of the structure is known and some parameters are assumed fromknowledge of the process by which the structure was made, leaving only afew parameters of the structure to be determined from the scatterometrydata.

By capturing higher diffraction orders such as the first orderdiffracted radiation, the novel spectroscopic scatterometer of FIG. 3,additional parameters for constraining the model or enhancing thelibrary can be used, improving the quality of metrology. The detectorsmay separately measure the intensity of transverse magnetic- andtransverse electric-polarized light and/or the phase difference betweenthe transverse magnetic- and transverse electric-polarized light.

FIG. 4 shows schematically the illumination optics 106 and zero orderdetection optics 108 in a little more detail. The source 104 is labeledS, and comprises for example a deuterium gas discharge source producinga broad spectrum of DUV radiation. Such sources are available forexample from Hamamatsu Photonics of Japan. Other types of source may beused if preferred, for example laser driven light sources (LDLS)available from a company Energetiq. An aperture plate labeled A definesan entrance aperture to the illumination optics. The aperture may be forexample 100×200 μm and elliptical in shape. A polarizer P1 selects onepolarization of light from the beam 140 and passes it to a focusingmirror M1 which focuses the polarized beam into a spot 164 illuminatingtarget 102. The illumination optics formed by mirror M1 forms ademagnified image of the aperture A, so that a small spot of diameterfor example 20 μm is formed on the target. Because of the oblique angleof incidence, the elliptical spot defined by aperture A is transformedinto a substantially circular spot on the target.

Specular reflection of the focused illumination spot 160 is the zeroorder reflected beam 144, which is captured by a second focusing mirrorM2 forming part of the detection optics 108. Detector 110 is shown in aparticular form having first and second pixel array detectors 110 a and110 b and a central focus detector 110 c. Mirror M2 is arranged to focusan image of spot 160 onto detector 110 that the location of focus sensor110 c. Also within the detection optics are a second polarizer P2 and adiffraction grating G. Polarizer P2 serves as an analyzer to blockradiation having an unwanted polarization. Diffraction grating G is inthis example a simple transmissive phase grating, rather than aconventional spectroscopy grating. Whereas a conventional spectroscopygrating would be designed with features such as “blazing” so as toconcentrate as much radiation as possible into a first order spectrum,the relatively simple phase grating used in this system produces a pairof symmetrically-arranged first order spectra of the zero order beam144. These spectra are labeled 146 a and 146 b in FIG. 4, and arecaptured by pixel array detectors 110 a and 110 b respectively. Aportion 162 of zero order radiation 144 passes directly through gratingG to form an image of spot 160 on focus detector 110 c. Assuming thedetection optics has a magnification similar to the demagnification ofthe illumination optics, the spot diameter on the focus sensor may be ofa similar size to the aperture A. Compared with the large NA (numericalaperture) of the known angle resolved scatterometer, the NA of theillumination optics and detection optics is significantly smaller and ofthe order of 0.1.

As shown by arrows 164 and 166, polarizers P1 and P2 can be rotated toselect any desired polarization. Different polarizations can be used fordifferent measurements, as mentioned above, enhancing the amount of dataavailable for analysis. A particular form of polarizer useful in thepresent apparatus is a Rochon prism, as shown at 166 in the insetdetail. A Rochon prism is formed by two pieces of birefringent material.For the wavelengths discussed in this example, a magnesium fluoridecrystal is a suitable. As shown in the inset detail, a ray coming infrom the left which is unpolarized is separated into its ordinary rayand extraordinary ray components. Specifically, the ordinary ray (o)which is polarized in the plane of the inset diagram passes straightthrough the prism without deflection, while the extraordinary ray (e),having a polarization into the page is deflected. Only the ordinary rayprogresses through the optical system. The extraordinary ray may beactively blocked, if desired, by a stop 168. The extraordinary ray canoptionally be detected by a detector (169, shown dotted). This could beuseful for example as a way to monitor variations in intensity of theoptical signals without stealing any of the wanted radiation.

The analyzing polarizer P2 can also be formed conveniently by a Rochonprism. The length of the Rochon prism in the optical path may be, forexample 20 to 30 mm.

In order to avoid confusion of terminology, it is useful to rememberthat both the target 102 and the grating G are periodic structuresacting as diffraction gratings. Thus, the beam entering the zero orderdetection optics illustrated at the right-hand side of FIG. 4 is thezero order beam, reflected by target 102. Within the detection opticsitself, grating G forms part of a spectrometer, and the spectra 146 aand 146 b comprise first order diffracted radiation of the differentialdiffraction grating G, while beam 162 impinging on focus detector 110 cis the zero order beam of the diffraction grating G.

In addition to having a very simple optical system, favorable to thevery short wavelength radiation in the UV region, producing the twospectra and focus beam 162 in the manner illustrated brings advantagesto the new design of spectroscopic scatterometer, as will be discussedbelow. These advantages are independent of the benefits brought byprocessing higher order radiation diffracted by target 102 (not shown inFIG. 4). A first benefit is that the position of the spot image on focusdetector 110 c varies according to how well spot 160 is focused ontarget 102. The focus detector may for example comprise a simple splitphotodiode. Deviation from ideal focus causes a deviation in theposition of the spot on the focus detector, which can be used in afeedback mechanism (not shown) to keep the spot is focused as possible.If desired, properties of the grating G can be tuned so that only asmall proportion of the radiation is allowed into this beam forfocusing, maximizing the energy in the first order spectra which are ofinterest for the metrology purpose. An actuator for this focusingmechanism may be arranged for example to move substrate W up and down torestore proper focus. An alternative arrangement of the optical systemswill now be described with reference to FIG. 5, in which focusing isfurther simplified.

Referring to FIG. 5, a rearranged version of the spectroscopicscatterometer shown in FIG. 4 has essentially the same components in adifferent layout. Components having the same reference numbers or labelsas those in FIG. 4 have substantially the same form and function.Differences may be in the magnification, angle of incidence and soforth. An optical path of the radiation in this modified arrangement isfolded so as to provide a more compact and vertically distributedapparatus. Such an arrangement can better fit in and instrument housing,particularly where it is to with place or sit alongside existingmetrology systems. In a practical instrument, it may be desirable tohave the new scatterometer available for some tasks, while known formsof spectroscopic and/or angle-resolved scatterometer are also provided,for other tasks. Folding of the optical path is achieved by additionalmirrors M3, M4, M5 as shown in the diagram. Sections of the optical pathbetween mirror M4 and M1 and between mirror M2 and polarizer P2 areparallel with one another, and normal to the plane of substrate W. Bythis arrangement, focusing of spot 160 onto the target can be controlledby an actuator 170 which simply moves up and down a subassemblycomprising mirrors M1 M2. In this way, the amount of mass to be moved bythe focus actuator can be much smaller than if the entire substratetable or the entire optical system of the scatterometer as to be movedto achieve focusing.

Also shown in FIG. 5, and optionally present in FIG. 4 arrangementlikewise, is a filter 172 which has a rotationally-symmetrictransmission filter with a gradually decreasing transmission in theradial direction. This filter ensures that sidelobes in the point-spreadfunction of the optics are significantly suppressed, resulting in lesslight “leaking” outside the target area. An example of such a filtersuitable for use at DUV wavelengths and its effects on the performanceof the scatterometer will be described further below, with reference toFIGS. 14 and 15.

An axis normal to the plane of the substrate is shown and labeled O inthe diagram. The angles of incidence and reflection of the zero orderbeam are labeled θ_(i) and θ_(r), respectively. The angle of incidencemay for example be 60° in a practical embodiment. The angle ofreflection is of course equal to the angle of incidence, for the zeroorder beam. Compared with the two-mirror arrangement of FIG. 4, thearrangement of FIG. 5 is clearly more complicated, and requiresadditional components. Nevertheless, refractive optical components areavoided, in favor of reflective optics. This has particular benefits forthe future generation of instruments, in which the radiation is in theDUV waveband.

Again, the first order diffracted radiation from target 102, and thefirst order detection optics 120 which are present in each of thedescribed embodiments, are not illustrated in FIG. 5, for simplicity. Aswill be explained further below, the apparatus in the examples describedbe configured and used in such a manner that the first order diffractedradiation is deflected out of the plane of the diagram, so that thefirst order detection optics do not need to share space with the zeroorder optics.

FIG. 6 illustrates the form of spectroscopic grating G in embodiments ofthe present invention, and further illustrates a technique used in theembodiments of FIGS. 4 and 5, to avoid crosstalk between spectra ofdifferent polarizations.

FIG. 6( a) is a plan view of phase grating G, with grating lines 180.FIG. 6( a) is a cross-section of the phase grating. The grating in thisexample is transmissive, and made from a glass or crystal that istransparent to the radiation in use, and has a certain refractive indexdifferent from the surrounding medium. Quartz may be used in the presentexample. Each grating line 180 comprises portions of differentthickness, in this case a simple rectangular wave pattern with period orpitch P and height h. Some of the incident radiation (here shownarriving at the back side of the grating) passes straight through thegrating to form a zero order beam 162. Other parts of the radiation arediffracted into first order beams 146 a, 146 b having angles dependenton the pitch P and wavelength λ.

The rays illustrated in FIG. 6( b) are parallel and monochromatic. Wherethe rays contain radiation of different wavelengths, the differentangles of diffraction cause the higher order beams to resolve intospectra. Where the rays are not parallel, some blurring of the spectracan be introduced, as discussed below. The relative strengths of thezero and higher order beams can be adjusted by tuning the height h ofthe grating. The tuning can in particular be used to accentuate thefirst orders, say, in a particular section of the wavelength range, andattenuate another section. This facility can be exploited as describedbelow, to compensate at least partially a non-uniform spectral responseof the light source and detectors.

Diffraction grating G can in other embodiments be a reflective gratingrather than a transmissive one as shown. The reflective grating can takethe place of one of the folding mirrors, for example mirror M5 in theexample of FIG. 5, or it may be an additional component, with suitableadaptation of the layout. The grating could be formed on a curvedmirror, but with a consequent increase in complexity. The gratings inthese examples are phase gratings, which have a greater efficiency(lower light loss) than amplitude gratings. Nevertheless, the apparatuswould work in the same way with an amplitude grating.

As mentioned already, the analyzing polarizer P2 may comprise a Rochonprism having a form similar to the polarizing prism 166 used for P1.Whereas the radiation leaving prism P1 is of a single, linearpolarization, interaction with the target 102 can cause a degree ofdepolarization, introducing an orthogonal component. In the second prismP2, depending on its orientation, this orthogonal component may beselected for analysis, or the original polarization may be selected.However, according to the function of the Rochon prism P2, both theordinary and extraordinary components (o-ray and e-ray) emerge from theprism, just in slightly different directions. Therefore without carefuldesign, it is likely that unwanted (e-ray) radiation will strike thegrating G and its spectrum could overlap the spectrum of the wanted(o-ray) radiation.

As illustrated in FIG. 6( a), the o-ray may be vertically orhorizontally polarized, and the e-ray will be polarized at ninetydegrees to that. In order that the spectra detected by detectors 110 aand 110 b contain only the wanted polarization, the grating G isarranged with its lines 180 at a different angle to the axes ofpolarization. This angle may be 45 degrees for convenience. The anglesthrough which the rays are spread out to form a spectrum areconsequently different to the angle through which the e-ray deviatesfrom the o-ray. As illustrated in FIG. 6( b), the detectors 110 a and110 b are similarly oriented at 45 degrees, so as to receive the o-rayspectra 146 a and 146 b. At the same time, because the e-ray isdisplaced either vertically or horizontally, with respect to the planeof the diagram, any e-ray spectra 146 a′, 146 b′, 146 a″ or 146 b″ aredisplaced out of the line on which the detectors lie.

Referring now to FIG. 7, the skilled person understands that aninstrument such as a spectroscopic scatterometer can only perform asdesired if there is, at the detector, sufficient intensity in theradiation across the spectrum of interest for useful measurements to bemade on a reasonable timescale. FIG. 7( a) illustrates the intensityspectrum of a commercially-available deuterium source 104 after theaperture A, converted to a photon count N_(A) representing the number ofphotons in a acquisition time. The acquisition time may be, for example,20 ms in a practical instrument. The source 104 in this example has a arelatively high intensity at shorter wavelengths (200 nm), which fallsto a substantially lower level at the longer end of the spectrum (400nm). FIG. 7( b) illustrates the net efficiency NE of an exampleillumination and detection optics, from the aperture A to the detector110. This represents the proportion of the photons that the reach thedetector at a given wavelength. This includes, for example, the loss of50% of the photons at the first polarizer P1, and assumes a certainreflectance at the mirrors M1, M2 etc, as well as a certain efficiencyof diffraction grating G. Importantly, the efficiency of the grating canbe tailored by design to be more efficient and those wavelengths wherethe source is weaker, so as to achieve a compensating effect in theshape of the net efficiency spectrum NE. Therefore, as illustrated inthis example, the efficiency spectrum can be selected to enhance thosewavelengths that are weaker in the source spectrum, relative to thosethat are stronger.

FIG. 7( c) illustrates the number N_(D) of photons at each wavelength atthe detector, resulting from the combination of incoming photons and netefficiency through the illumination and detection optics. It will beseen that the number of photons at the detector is still substantiallyhigher at the lower wavelengths than at the longer wavelengths. However,this fall in photon count is matched by a corresponding rise in thesensitivity S_(D) of detector 110, to achieve a relatively flat responsefor the instrument as a whole. The response will never be perfectlyflat, of course, but can be modeled and/or measured to produce acorrection curve to be applied within processing unit 130. FIG. 8illustrates the phenomena of aberration, that arises in the formation ofthe spectra 146 a and 146 b due to the very simple nature of thedetection optics. A spot 190 with diameter D is formed on the focusdetector from the zero order beam 162 of grating G. A first orderspectrum (for example, spectrum 146 a) is formed by rays deflected bythe action of the grating, which has a period P. The angle θ at whicheach wavelength λ in the spectrum is deflected depends on bothwavelength and grating period, as shown.

Whereas, in a conventional spectrometer, came be taken to ensure thatthe rays arriving at the grating were parallel, in this simple opticalsystem, the rays are converging as shown to the spot 190. Consequently,they are not parallel, by an amount dependent on the numerical apertureNA. Consequently, for each wavelength in the diffraction spectrum, therays do not focus perfectly to a spot, and the spot 192 is ratherblurred due to aberrations. Detector 110 a is placed at an angle, shownin dotted outline, so as to obtain the best approximation of a focusedspectrum. The aberration can be modeled as illustrated in FIG. 9, givingan uncertainty Δλ in the wavelength that will be reported by thespectrometer based on a design with parameters approximately NA=0.01,L=100 mm, D=200 and P=200 μm. This amount of blurring of the spectrum iswithin acceptable limits for a useful instrument. Moreover, because thecause of the blurring can be very well modeled, the amount of blurringcan be calculated so processor 130 can apply deconvolution to themeasured spectra, and further improve accuracy.

FIG. 10 illustrates some benefits of the novel arrangement, in whichsymmetrically opposite spectra are recorded simultaneously on the twodetectors 110 a and 110 b. Each detector may, for example, comprise aCCD array having 1024 pixels by 16 pixels. Each pixel position in thelonger direction represents a different point on the spectrum. Intensityis measured by pixels across the shorter direction of the array areintegrated into a single value. Example spectra as seen by the detectorsare represented by the curves I_(a)(λ) and I_(b)(λ). It will be noticedthat the spectra of mirror images of one another, centered on the focusdetector 110 c, as would be expected. Values from both sensors can becombined (averaged) to obtain a measurement of the intensity of thespectrum at a given wavelength, according to the formula

I(λ)=(I _(a)(λ)+I _(b)(λ)/2

An important performance parameter for a spectroscopic scatterometer isthe robustness of the reported spectrum against errors caused by offsetsin the focus of the spot 160 on the target 102. In general, an error ΔZin focus will produce an error Δλ(ΔZ) in the accuracy of the reportedwavelength. Therefore, in known spectroscopic scatterometers, it isnecessary for focus to be very tightly controlled to minimize theoffsets. On the other hand, in the symmetrical design used in thepresent embodiments, focus offsets have much less effect on the reportedwavelength. To understand why this is the case, it should be noted thatfocus offsets will cause a shift Δλ(ΔZ) of both spectra in the samedirection, while the reported wavelength varies in opposite directionson the two sensors. Thus, the shift to longer wavelengths in thespectrum reported by detector 110 a is compensated by a shift to shorterwavelengths in the detector 110 b. Consequently, while defocus leads toa blurring of the reported spectrum, there is no shift in the wavelengthdirection caused by focus errors. Consequently, performance parametersof the focusing system can be relaxed, leading to a simpler constructionand/or faster measurement performance. If desired, because theadditional spectral blurring caused by defocus is easily modeled,processor 130 can take this into account using a focus error signal fromthe focusing system.

In addition to providing a focus error signal, detector 110 c can beused to measure variations in the overall intensity of the illumination,so as to correct for such variations before processing the measuredspectra. This avoids the need to provide a separate sensor elsewhere inthe system.

As a further benefit of the symmetrical design of the spectroscopicscatterometer, information can be obtained on wavelength-dependentvariations in the apparent focusing position of the scatterometer(apparent surface depression or ASD). This in turn can be useful topredict process-dependent ASD effects that influence the measurement ofsubstrate height during exposure of a subsequent pattern. An exampleapparatus and method exploiting this potential will be illustrated anddescribed below with reference to FIGS. 16 and 17.

FIG. 11 illustrates a simple device 200 useful in the calibration of aninstrument such as the spectroscopic scatterometer described above. Aparticular requirement is for the spectral response of the completeinstrument to be measured and stored for correction purposes, before thespectral response of actual targets 102 can be accurately measured andcalculated. When moving to shorter wavelengths, such as DUV wavelengths,the availability of reference reflectors becomes a problem. At suchwavelengths, conventional reflecting materials exhibit their ownwavelength-dependency. Device 200 comprises a prism of a highlytransparent material such as quartz or fused silica, that does notexhibit great wavelength dependence in the waveband of interest. Thepresent can be located on the substrate support WT of the instrument,such that an incoming ray 201 is reflected by total internal reflectionat a face 202 to give an outgoing ray 203. Entry face 204 and exit face206 are performed so that they are completely normal to the direction ofthe incident ray 200 and one or 203, and no deviation occurs. The entryand exit faces can optionally be given a small curvature that matches aconvergence or divergence of the incident and reflected beams. In thisway the light rays “hit” the faces perpendicularly so the beamdistortion that is caused by the faces is minimized.

The intensity I_(R) of the outgoing ray 203 is related to the intensityI₀ of the incoming ray 201 by a reflectance factor R which is known withvery high accuracy, and depends only on the refractive index of thematerial from which the prism is made. The formula for reflectance R is

R=t² a

where a is the absorption of the prism material and t is thetransmission at each of the faces 204, 206. Provided the refractiveindex n is well known at each wavelength, the transmission t can becalculated from Fresnel's equation

t=4n/(1+n)²

The calibration process whereby spectral response of the completeinstrument, which would otherwise be a challenging, is simplified by theuse of this accurate reference device. For calibrating an instrumentwith different angles of incidence, a different prism can be made.Alternatively, a single prism could be made that has facets at differentangles, if preferred.

FIG. 12 illustrates the layout of a practical apparatus comprising twoof the spectroscopic scatterometers of the type shown for example inFIG. 4 or FIG. 5, arranged at right angles to one another Eachscatterometer will be referred to as a branch of the completeinstrument, and these branches are labeled branch A and a branch B forconvenience. Remembering that the target 102 has the form of adiffraction grating in a typical situation, the lines of this gratingare shown oriented vertically in the diagram, that is parallel to the Yaxis. The scatterometer branches A and B are oriented at right angles toone another such that both are at 45° to both the X and Y axes. Thus theincoming beam of branch A travels schematically from a source SA atbottom left, reflects from the target and enters detection optics (notshown) at the upper right portion of diagram, while the incoming beam ofbranch B travels from a source SB at the bottom right, reflects from thetarget and enters detection optics at the upper left portion of thediagram.

Providing two independent scatterometer branches and placing them atright angles to one another, and at 45° to the target grating enablesenhanced functionality as will now be explained. Firstly, by measuringthe same target grating from two different directions, measurementresults from the branches A and B can be compared in various ways. Inone example measurement process, the two branches are used withdifferent polarizations of incoming radiation, which avoids the need toswitch polarizations to obtain measurement of a target a differentpolarizations. Throughput of the instrument can be greatly increased,because switching polarization generally requires a mechanical changewithin the instrument, with associated delay and settling time. Inanother application, with the same polarization is used in bothbranches, the zero order spectra of the target grating taken fromdifferent directions can be compared to obtain information on theasymmetry of the grating structure. This can be used for example tomeasure overlay error of structures fabricated at the resolution limitof the lithographic cell. These are just two examples of the increasedfunctionality provided by the two spectroscopic scatterometers workingin parallel as branch A branch B.

In addition to the functionality just described, provision of one or twospectroscopic scatterometers whose zero order path is at an angle to thelines of the target grating 102 enables higher order scatterometryspectra from the target to be detected at a location which is outsidethe plane of the zero order path. Referring again to FIG. 12, incidentradiation from branch A arrives generally in the direction shown byarrow A′, while the zero order radiation enters the detection optics ofbranch A in the direction shown by arrow A⁰. First order diffractedradiation from the target leaves at an angle indicated by dotted arrowA⁻¹, according to a well-known principle of “conical diffraction”.Similarly, zero order and first order diffracted radiation for branch Bis following the direction indicated by arrows B⁰ and B⁺¹, respectively.

Referring again also to the general layout indicated in FIG. 4, it willbe appreciated that the first order detection optics 120 for each branchof the scatterometer are located so as to intercept the beams followingthe directions A⁻¹ and B⁺¹. Moreover, as will now be illustrated withreference to FIG. 13, the proximity of the first order beams from boththe A and B branches allows the detection optics for the first orders ofboth branches to be combined using shared components.

FIG. 13( a) is a schematic plan view of the first order detection optics120 in the instrument just described, while FIG. 13( b) is a schematicside view of the same detection optics, looking along the X axis. Eachview (a) and (b) is only partial, for reasons of clarity. Suffix ‘A’ or‘B’ indicates a component belonging to the first order detection opticsof branch A or branch B. Where the suffix is absent, the view is genericto both branches. Suffix ‘a’ or ‘b’ indicates components associated withone of two opposite spectra, similar to the use of suffix a or b inFIGS. 4 to 6. Again, where the suffix a or b is absent in this textand/or in the drawing, the feature is generic to both spectra. A firstcomponent of the first order detection optics 120 is an ellipticalmirror segment 210 this collects first order diffracted beams 150A and150B from both the A and B branches of the instrument, and focuses themin the direction of detector 122. In the path of each of these beams, afurther diffraction grating to 212 is provided to spread the first orderbeam into a spectrum, the spectrum for each branch A and B being labeled152A and 152B, respectively.

It will be appreciated by the reader that radiation in the first orderdiffracted beam 150 is itself a spectrum, spread according to therelationship between a pitch of the target grating and a wavelength λ ofthe illumination. Following three rays of different wavelengths asdrawn, it will be seen that each passes through grating 212 at adifferent place. Now, taking the spectrum of a spectrum does not providea better spectrum. However, the design of the first order detectionoptics is such that the spectroscopic effect due to the target gratingitself is cancelled out, by the time the rays reach the detector 122.This is simply achieved by ensuring that, but for the presence of thespectroscopy grating 212, mirror 210 would focus all the first orderradiation from spot 160 on the target, to a spot image 160′ in the planeof detector 122. Accordingly, when grating 212 spreads the radiationinto a pair of spectra 152 a and 152 b, detectors 122 a and 122 breceive spectra determined only by the spectroscopic effect of grating212, and independent of the spectroscopic effect of the target grating.There is therefore no need to focus the first order radiation into apoint first, and then feed it to a separate spectrometer.

As in the case of the zero order branch, the grating can be madetransmissive or reflective, and can be a phase grating or amplitudegrating. In the case of a reflective grating, the spectroscopy gratingcan be made for example on the surface of the elliptical mirror 210.(The geometry of the lines would in that case be a distorted version ofthat shown in FIG. 13( c), but this can be calculated and formed by forexample photolithographic techniques.)

As seen in the side view, the ‘plane’ of detector 122 can be madeapproximately curved, to compensate for aberration in the focus of thedifferent parts of the spectra 152 a and 152 b. This aberration isexactly the same as that described above, with reference to FIGS. 8 to10, for the zero order detection optics. As before, the resultantblurring of the spectra can be simply modeled and corrected, if desired,by signal processing.

As shown in FIG. 13( c), first order spectroscopy grating 212 has linesarranged at 45°, similar to the zero order grating illustrated in FIG.7( a). Separate grating portions 212A and 212B are provided for the raysfrom the two branches A and B, with opposite 45° orientations. FIG. 13(d) illustrates a set of four detectors 122Aa, 122Ab, 122Ba, 122Bb whichcollect the symmetrical spectra of the first order radiation for thebranches A and B, in a manner similar to the operation of the zero orderdetection optics, described above with reference to FIGS. 7 to 10. Acentral detector 122 c is optional, but could be arranged for example tomeasure the intensity of the zero order of the first order beam, and/orto provide an independent focus signal for the first order detectionoptics. Independent focusing of the higher order detection optics islikely to be an unnecessary complication in practice, as the first orderdetection optics can be designed and manufactured to have the same focalpoint as the zero order detection optics. In this completely symmetricalarrangement, the central detector 122 c, if provided, would receivebeams from both branches A and B, however, and this would need to betaken into account in the design and operation of such a detector.

Because the grating 212 used to form the higher order spectrum isdistinct from the target grating, its performance can be designed toachieve a desired spectral resolution, and one which is independent ofthe target grating pitch. This is in contrast to the known MOXIEproposal, in which the target grating is relied upon to perform thespectroscopy.

In conclusion, a spectroscopic scatterometer apparatus has beendescribed with many distinctive and useful features. In addition toanalyzing a spectrum of zero order radiation reflected from target,first order diffracted radiation is also captured and its spectrum isanalyzed separately. Results of these analyses are combined by numericalprocessing, to obtain measurements of various properties of the target.In obtaining the spectra from the zero order and/or higher orderradiation, a novel spectrometer design based on a simple phase gratingcan be implemented, in which two symmetrical spectra are detected andcombined to obtain a measured spectrum. The combined spectrum isrelatively insensitive to focus variations, while a zero order beamthrough the phase grating can be used directly as a focus sensor.

Other particular features of the examples described bring benefits inthe implementation and/or performance of the instrument. For example,placing a spectroscopy grating at an angle to the polarization of theradiation being analyzed allows a spectrum to be obtained withoutcrosstalk between ordinary and extraordinary rays. As another example,placing and optical path of the instrument in a plane which is at anangle to an orientation of the target grating 102, higher orderdiffracted radiation becomes diverted out of that plane, when it may bemore easily captured by higher order detection optics. By providingorthogonal branches of the scatterometer, multiple measurements of thesame target grating can be made simultaneously. For example, these maybe made with different polarizations, or they may be made with the samepolarization, to obtain information on asymmetry of the target grating.In practice, these techniques can be combined and each target to obtaina full set of measurements. For each polarization of incident radiation,the analyzing polarizer P2 can be rotated to 2 different orientations,giving a total of four measurements to discover thepolarization-dependence of the scattered spectrum.

Higher order spectra from multiple branches of the instrument can becollected and processed to add to the spectrum information available forreconstruction. With careful design, optical components can be sharedbetween the higher order detection optics of the different branches.First order and higher order diffracted spectra can be obtained. Inprinciple, an analyzing polarizer could be included in the higher orderbeam prior to the detector 122, similar to polarizer P2 in the zeroorder branch. However, the lack of a polarizer makes the optics simpler.In the first order branch the light is already dispersed in differentangles by the metrology target. As a result, the angular divergence ofthe light is significantly larger as in the case of the zero orderbranch. This restricts the types of polarizer that could be used. In theUV waveband there are currently no polarizers that can deal with suchdiverging light beams.

Referring to the measurement of asymmetry in the target grating, thiscan be extended to a measurement of overlay error, provided thatmaterials between the two layers whose overlay is being measured aresufficiently transparent to the radiation being used.

The above and numerous other applications of the apparatus will beapparent to the skilled reader and the present invention is not limitedto application in any particular measurement. Numerous modifications andvariations of the apparatus can be envisaged within the competence ofthe skilled person. As has been mentioned already, for example,embodiments of the present invention may collect diffracted radiation oforders higher than the first order, and obtained spectra for suchradiation to be processed in a reconstruction or other metrology task.While the arrangement having two identical scatterometers arranged atright angles to one another provides particular versatility, it is notessential that these scatterometers are identical, if one would preferthem to have different capabilities. It is not essential, for example,that they both have higher-order branches. Where the two scatterometersdo have higher-order branches, it is not necessary for them to sharecomponents such as the elliptical mirror 210, if another arrangementwould be more practical or provide higher performance. Where a certainnumber of optical components such as the mirrors M1 to M5 and theelliptical mirror section 210 have been illustrative, optical systemshaving a greater or fewer number of elements can of course be designed.For example, although it is not believed necessary to achieve a goodperformance in the currently-intended applications, the blurring of thespectrum due to aberration could be reduced by a more sophisticateddetection optics. The skilled reader will appreciate that a trade-offexists, as always, between improving performance in some respects, andintroducing degraded performance and/or increased cost by seeking a moresophisticated arrangement.

FIG. 14 illustrates in more detail a filter included in the illuminationoptics that produces the spot by imaging a source. In the apparatus ofFIG. 5, for example, the filter is the filter 172, the source isrepresented by aperture A. The illumination optics includes focusingelements illustrated as a lens but implemented in this example in theform of mirror M1. The focused illumination spot is labeled 160. Inspectroscopic scatterometry it is advantageous if all the measurementlight is projected inside the metrology target (in other words: the spotshould to “underfill” the grating). Any stray light outside themetrology target can potentially lead to metrology errors, particularlyif product features or other targets are present in those areas.

Without the filter 172, diffraction means that the point spread functionI_(PSP) of the optics that images the illumination spot on the wafer isa rotationally symmetric Airy function:

${I_{PSF}(r)} - \left\lbrack \frac{2{J_{1}(r)}}{r} \right\rbrack^{2}$

Here J₁ is a Bessel function and r is a normalized radial coordinategiven by:

$r = {a\frac{\pi \times {NA}}{\lambda}}$

In this expression, NA is the numerical aperture of the imaging opticsand λ is the wavelength of the light and a is the radius from the centerof the Airy function. Each point of light in the source aperture A willbe spread in accordance with this function. Even in a case where theaperture is made very small to obtain a small illumination spot, thespot will be spread by diffraction to have sidelobes in accordance withthis Airy function. The point spread function in turn dictates thespatial frequency response of the illumination optics, represented bythe modulation transfer function MTF.

The Airy function has a central peak and relatively strong oscillatorysidelobes that decay only slowly as a function of radius. The relativelystrong and slow decay of the sidelobes result in a relatively largeamount of the light this imaged outside the metrology target. This isparticularly significant in spectroscopic instruments, where a lowillumination NA is generally used.

As shown in FIG. 14, filter 172 has a transmission that decreases withdistance a from the optical axis O. We shall refer to it as a radialtransmission filter. As shown in the inset detail, filter 172 may bemade for example of a metal-coated thin plate 172 a with a metal layer172 b of radially increasing thickness h. Example parameters of a filtersuitable for UV wavelengths are given below. While the filter is shownas a separate component in this example, it can also be integrated withanother element, for example by applying the coating to the mirror M1.

The filter is placed in or near a pupil plane of the illuminationoptics, so that its effect is to gradually attenuate higher spatialfrequencies. This results in a reduction of the sidelobes of the pointspread function I_(PSF). Analogous behavior is well-known from signalprocessing theory where filters are used to reduce “ringing”. It isapplied here in scatterometry to minimize the amount of the light thatleaks outside the metrology target.

In general, any gradual decrease in transmission will give a reductionin sidelobes. However, specific radial variations will give a veryefficient reduction. A well-known example from signal theory is theHanning window function which gives a transmission function T(a) definedby:

${T(a)} = {\frac{1}{2}\left\lbrack {1 + {\cos \left( {\pi \frac{a}{a_{MAX}}} \right)}} \right\rbrack}$

where a is the radial coordinate as shown in FIG. 14 above.

FIG. 15( a) shows the shape of this Hanning window. As alreadymentioned, this transmission function can be realized in a practicalfilter by depositing a metal film on a substrate with a radially varyingthickness. This thickness distribution shown in FIG. 15( b) can, forexample, be made by first depositing a 50 nm thick Ti film on asubstrate. This film is then radially etched away by gradually closing acircular diaphragm in an etch chamber. In that case the center has thelongest etching time so the Ti film is completely removed. The edge hasthe shortest etch time so the thickness is not reduced at the edge.Other fabrication methods are, however, also possible. In the example ofFIG. 15( b), the metal thickness increases slowly at first and then atan increasing rate with increasing distance a from the center, with theaim that a radial transmission function of the filter approximates theHanning function.

FIG. 15( c) shows the calculated radial transmission function T forthree different wavelengths for the thickness distribution shown in FIG.15( b). T in this notation represents the transmitted intensity (i.e.power), expressed as a fraction of the incident intensity. As can beseen there is some variation in attenuation for the differentwavelengths. If necessary, the wavelength variation can be furtherreduced by mixing several materials or by depositing several films.This, however, increases complexity and may not be needed in practice.

FIG. 15( d) is a graph of the calculated PSF of the illumination opticsM1, M2 without (NF) and with (F) the radial transmission filter 172.These functions have been calculated for radiation of λ=300 nm andfocused with an NA=0.1 on a substrate with 60° angle of incidence. (Theoblique angle of incidence gives the spot an elliptical shape, and thegraphs show the cross-section of the spot on its minor axis). Bearing inmind the vertical log scale, we clearly see that the radial transmissionfilter will give a very significant reduction of the diffraction rings.

A drawback of the filter is the loss of light, which would lead toincreased noise or longer measurement acquisition times. However, theeffective NA is also reduced so one could increase the NA of thefocusing optics to a level where the effective NA is again almost equalto the originally desired NA. In this way the light loss is probablylimited to acceptable levels.

FIGS. 16 and 17 illustrate the phenomenon of apparent surface depressionor ASD, and the manner in which it may be measured using the symmetricalspectra detected in the spectroscopic scatterometer apparatusesdescribed herein.

In lithography using an apparatus such as that shown in FIG. 1, thewafer height is optically measured with a level sensor. The operation ofthis level sensor typically is based on optical triangulation. A patternis projected on the wafer at an oblique angle and this reflected patternis re-imaged on a detector. A height variation on the wafer will resultin the shift of the pattern on the detector. This is results in ameasured height variation. The height variations are mapped across thesubstrate W prior to exposing the substrate to the patterned beam usingprojection system PS. Errors in height measurement leads to poorfocusing of the desired product pattern and therefore limit theperformance of the lithography step as a whole.

FIG. 16( a) shows a typical substrate W undergoing level sensing. Anunderlying Si substrate 1600 is coated with radiation sensitive resist1602, with an intermediate anti-reflection layer 1604. The layerthicknesses are not shown to scale. It will be appreciated that in themajority of lithographic steps, other layers previously applied andperhaps patterned may lie between layer 1604 and the ultimate substrate1600. Incoming radiation from the level sensor is shown at 1606 andreflected rays are shown at 1608. Ideally, the sensor should measure thetop of the resist surface. Typically the known level sensors works withvisible light and resist and BARC layers are transparent for themeasurement light. Moreover, layers that may lie beneath the BARC layerare also transparent for the measurement. As a result, the measurementlight penetrates the stack of resist and process layers, resulting inthe illustrated measurement error which is called the Apparent SurfaceDepression or ASD. In practice this ASD effect can become very large(>100 nm)

The real problem, however, is not so much the magnitude of this errorbut the fact that it can vary from wafer-to-wafer and from lot-to-lot.Therefore it cannot be corrected without information specific to eachwafer, which is costly in terms of throughput. As a first step toaddress to reduce these variations a level sensor in future may use UVinstead of visible light. In case of the UV LS the measurement light isabsorbed in the top layers of the stack so the resulting processsensitivity due to stack variations will be significantly reduced.However, the ASD will still be a problem for the UV level sensor for afew types of process layer that are partially transparent even in the UVrange. In practice, it is not always possible to predict beforehandwhether the UV level sensor will be sufficiently effective, or ifleveling problems are still due to residual ASD variations. In order toassess if leveling problems are due to residual ASD variations it isdesirable if a measurement system were available that could detectvariations in the LS process dependency.

It may be noticed that the oblique incidence and reflection of the rays1606 and 1608 in FIG. 16( a) is similar to the oblique incidence andreflection of illumination and diffracted rays 142, 144 in thescatterometer of FIG. 5. The inventor has recognized that the novelspectroscopic scatterometer disclosed herein can be used instead or inaddition to detect residual ASD variations. This information canoptionally be used to derive a correction signal that can be fed forwardto the leveling system.

FIG. 16( b) illustrates a reflectance spectrum of a target substratemeasured by the scatterometer. The reflectance R is simply the intensityI_(R) of the detected radiation, divided by the intensity I₀ of theincident radiation. All three quantities these vary as a function ofwavelength λ. As already described in relation to FIG. 7, it needs to betaken into account that the sensitivity of the detector varies withwavelength also. The apparent surface depression ASD is also a functionof wavelength, and FIG. 16( c) shows the calculated ASD for a typicalstack.

Referring to FIG. 17, we see again the detector layout of thespectroscopic scatterometer that can be used to measureprocess-dependency effects of the level sensor in a lithographic system.The symmetrical spectra are shown in graphs above each detector. Thespectra in this figure have been converted already to reflectance R_(a)and R_(b), while the spectra in FIG. 10 were shown in their intensityvalues I_(a) and I_(b). The exact operation of this scatterometer invarious embodiments has already been discussed above in relation toFIGS. 4 to 13 and will not be repeated in too much detail here. Inprinciple, however, what we describe here is an independent instrumentfor obtaining information about ASD. The fact that the same hardwarealso forms the basis of a spectroscopic scatterometer capable of manyother measurements is something that the designer and user of theapparatus may exploit or ignore as they wish.

Recalling FIGS. 4 and 5, the aperture A is imaged on the substrate W viamirror M1. The second mirror M2 is used to image the measurement spot160 onto detector arrays 110 a and 110 b and the focus sensor 110 c.Because of phase grating G, the signal measured on detectors 110 a 110 bis the wavelength spectrum of the reflected light. Since we illuminatethe target at an oblique angle, the location of the spectrum on thedetectors D1 and D2 is very sensitive to wafer height errors. Asdescribed already above, we monitor and control the wafer height veryaccurately with the focus sensor 110 c and focus actuator 170. Themeasured wafer height, however, is wavelength dependent. Thiswavelength-dependency we can measure using the symmetrical spectraobtained by detector arrays 110 a, 110 b.

Referring now to FIGS. 16 and 17 together, the phenomenon of ASD meansthat the reflected spot in the scatterometer, as measured by focussensor 110 c, lies beneath the true position of the top of theresist-air interface. However the ASD effect varies in magnitude withthe wavelength. As a result of this, the spectra on the detector arrays110 a and 110 b will show a small but significant wavelength-dependentshift as indicated by the small arrows on the spectra of FIG. 17. In thex-coordinate system, this shift has the same direction. By contrast, thewavelength (λ) coordinate on the two detector arrays 110 a and 110 b,points in opposite directions. As a result, the direction of theASD-induced shift of the spectrum is opposite on detectors 110 a and 110b, and in their spectra R_(a) and R_(b).

This symmetry effect was exploited above to obtain a spectrum using theaverage of the two spectra, making the spectrum less sensitive to focuserrors. The inventor has recognized that the same symmetry effectenables us to obtain information about the ASD effect at a givenlocation on a substrate using the difference of the two spectra. Recallthat the two spectra should be identical, and that a general focus errorresults in a complete shift of the spectra as illustrated in FIG. 10. Itcan be shown that a difference signal between the detected spectra ateach wavelength is related to the wavelength-dependent ASD by thefollowing relationship:

$\frac{R_{a} - R_{b}}{2} \propto {\frac{R}{\lambda} \times A\; S\; {D(\lambda)}}$

According to this equation, we cannot detect ASD effects in the areas inthe spectrum where dR/dλ=0 so there will be some blank regions in themeasured ASD spectrum. It is expected, however, that these areas arelimited. If necessary one can interpolate ASD values for these regions,or one could shape the source radiation spectrum to add a gradient tothe illumination intensity I₀(λ) in regions where dR/dλ=0. This islikely to be an undesirable complication, however.

The measured ASD information can be used in various ways. The mostinteresting application area is probably to use this information tofurther improve the robustness of the level sensor. For example, thismethod can be used to measure the ASD effect on the wafer before itenters the exposure tool. In any case, the capability to detect ASDvariations is a useful feature of the novel spectroscopic scatterometerhardware. It will be understood that the ASD spectrum measurement can beadded to the capabilities of the described spectroscopic scatterometerby a simple addition to the software in processor PU (not shown in FIGS.4 to 13 but provided in each case).

FIG. 18 is a flowchart illustrating the application of ASD measurementto improve imaging in a lithographic process. At 1800 the substrate isloaded into a spectroscopic scatterometer, which can be separate from alithographic tool. The scatterometer is operated (1802) as describedabove to obtain measurements of ASD at one or more wavelengths, and atone or more positions. Of particular interest is any wavelength orwavelengths used in the level sensor of the lithographic tool. The ASDmay be measured for example at inner an outer portions of the substrate,to account for differences in layer thicknesses due to spin coating orprocess steps like chemical mechanical polishing (CMP). At 1804 thesubstrate is loaded into the lithographic tool in preparation for apatterning operation. The tool may be of the type shown in FIG. 1 forexample. At 1806 the tool uses its level sensors LS to obtain a heightmap of the substrate surface. At 1808 a correction is applied to themeasured height map, based on the ASD information previously obtainedusing the scatterometer. (The correction could if preferred beprogrammed into the level sensor while it operates; the result is thesame).

At 1810 the substrate is exposed as described in relation to FIG. 1,applying a pattern from patterning device MA by imaging it onto eachtarget portion C of the substrate W. Because the height map used forfocusing during this imaging has been corrected to account for ASDeffects that are specific to this substrate, the image quality isimproved. At 1812 the substrate is processed as described with referenceto FIG. 2 above, to develop the pattern exposed in the resist layer.Depending which particular product layer is being patterned, theprocessing either concludes at 1614 or returns at 1616 for further stepsof coating, exposing, etching and so forth.

Further embodiments according to the invention are provided in belownumbered clauses:

1. Inspection apparatus comprising a spectroscopic scatterometer having:

illumination optics for directing broadband radiation with an angle ofincidence at a spot on a target structure, the target structure in usecomprising a periodic grating;

zero order detection optics for receiving radiation reflected from thetarget and for forming and detecting a spectrum of the reflectedradiation; and

higher order detection optics for receiving radiation diffracted at oneor more higher orders by the periodic grating in the target structure,and for forming and detecting a spectrum of the received diffractedradiation.

2. Inspection apparatus according to clause 1 wherein the higher orderdetection optics comprise:

a focusing arrangement for focusing the higher order diffractedradiation emanating from the spot into an image spot comprising allwavelengths of radiation,

at least one higher order spectrum detector positioned to at least oneside of the image spot, and

a diffraction grating positioned in an optical path between the spot andthe image spot so as to form a spectrum of the higher order diffractedradiation on the higher order spectrum detector.

3. Inspection apparatus according to clause 2 wherein the diffractiongrating is symmetrical so as to form a symmetrical pair of spectra ofthe higher order diffracted radiation, and a pair of spectrum detectorsare provided so as to capture both of the pair of spectra, the apparatusfurther comprising a processor for combining measurements from bothdetectors to obtain a single detected spectrum.

4. Inspection apparatus according to clause 2 or 3 wherein thediffraction grating comprises a phase grating of either transmissive orreflective type.

5. Inspection apparatus according to clause 2, 3 or 4 wherein thefocusing arrangement comprises a single curved mirror.

6. Inspection apparatus according to any preceding clause wherein theillumination optics and zero order detection optics process radiationgenerally in a first plane perpendicular to a plane the targetstructure, while the higher order detection optics are arranged toprocess radiation in a second plane, angled relative to the first plane,the higher order diffracted radiation entering the second plane duringuse of the apparatus as a result of an oblique orientation of lines theperiodic grating in the target structure, relative to the first plane.

7. Inspection apparatus according to clause 6 wherein the first plane isset at an angle of 45° relative to the expected orientation of the linesin the periodic grating.

8. Inspection apparatus according to clause 6 or 7 wherein a secondspectroscopic scatterometer is provided for simultaneous measurement ofthe same target structure, a first plane of the second spectroscopicscatterometer being arranged at right angles to the first plane of thefirst-mentioned spectroscopic scatterometer.

9. Inspection apparatus according to clause 8 wherein one or moreoptical components are shared between the higher order detection opticsof the first-mentioned and second scatterometers.

10. An inspection apparatus according to any preceding clause arrangedfor operation with radiation at wavelengths shorter than 400 nm, whereinthe illumination optics comprise one or more curved mirrors for focusingthe radiation into a spot on the target structure, while the zero andhigher order detection optics each comprise one or more curved mirrorsfor focusing an image of the spot at a point to one side of a spectrumdetector.

11. Inspection apparatus comprising a spectroscopic scatterometerhaving:

illumination optics for directing broadband radiation with an angle ofincidence at a spot on a target structure;

detection optics for receiving radiation diffracted at a zero or higherorder from the target structure and for forming and detecting a spectrumof the diffracted radiation,

wherein the detection optics comprises a symmetric diffraction gratingarranged to form a symmetrical pair of spectra of the reflectedradiation, and wherein a pair of spectrum detectors are arranged todetect both of the spectra, the apparatus further comprising a processorfor combining measurements from both detectors to obtain a singledetected spectrum of the reflected radiation.

12. Inspection apparatus according to clause 11 wherein a furtherdetector is arranged to receive a zero order beam of the grating, thefurther detector being located at a point generally in between the pairof spectrum detectors, and wherein signals from the further detector areused for monitoring focus of the spot on the target structure.

13. Inspection apparatus according to clause 11 or 12 comprising firstand second spectroscopic scatterometers of generally similar formarranged for simultaneous measurement of the same target structure,wherein the illumination optics and detection optics of eachscatterometer are arranged to process radiation generally in a firstplane perpendicular to a plane of the target structure, and wherein afirst plane of the second scatterometer is arranged at an angle to thefirst plane of the first-mentioned scatterometer.

14. Inspection apparatus according to clause 13 wherein the first planesof the first and second spectroscopic scatterometers are arranged atright angles to one another.

15. Inspection apparatus according to any of clauses 11 to 14 whereinthe detection optics is arranged to capture zero order diffractedradiation, being radiation reflected from the target structure, thedetection optics including an analyzing polarizer in a path of thereflected radiation prior to the phase grating, the analyzing polarizercomprising a Rochon prism arranged to transmit ordinary andextraordinary rays in different directions thereby to select only onepolarization for the formation of the spectra, and wherein the phasegrating has lines oriented at an oblique angle relative the selectedpolarization, whereby radiation of a different polarization will haveits spectra formed by the phase grating at a location away from the pairof spectrum detectors.

16. Inspection apparatus according to any of clauses 11 to 15 whereinthe detection optics comprise a focusing arrangement for focusing thediffracted radiation emanating from the spot into an image spot at alocation generally in between the spectrum detectors, the diffractiongrating being positioned in an optical path of the focusing arrangementbetween the spot and the image spot so as to form the spectra of thediffracted radiation on the spectrum detectors.

17. Inspection apparatus according to clause 16 arranged for operationwith radiation at wavelengths shorter than 400 nm, wherein the focusingarrangement comprises one or more curved mirrors.

18. Inspection apparatus according to any of clauses 11 to 17 furthercomprising means for processing the spectrum to correct for anaberration caused by rays of the reflected radiation being non-parallelat the location of the phase grating.

19. Inspection apparatus according to any of clauses 11 to 18 whereinthe diffraction grating is a transmissive phase grating.

20. Inspection apparatus according to any of clauses 11 to 18 whereinthe diffraction grating is a reflective phase grating.

21. Inspection apparatus according to clause 20 wherein the diffractiongrating is formed on a curved mirror that serves also for focusing thediffracted radiation emanating from the spot into an image spot at alocation generally in between the spectrum detectors.

22. A method of measuring properties of a target structure on asubstrate, a target structure including a periodic grating, the methodcomprising obtaining a zero order spectrum and at least one higher orderspectrum from the target structure using an inspection apparatusaccording to any of clauses 1 to 10, and processing the measured spectraso as to obtain measurements of one or more parameters of the targetstructure.

23. A method according to clause 22 wherein the zero order and higherorder spectra are measured for the same target more than once, usingdifferent polarizations of incident radiation and/or selecting differentpolarizations in one or both of the zero order detection optics andhigher order detection optics.

24. A method according to clause 22 or 23 wherein the zero order andhigher order spectra are measured for the same target using differentangles of incidence of radiation relative to an orientation of theperiodic grating in the target structure, and wherein measurementsobtained from the different angles of incidence, combined to measure anasymmetry property of the target structure.

25. A method according to clause 24 wherein the spectra are measured inparallel using different angles of incidence by providing twospectroscopic scatterometers with different orientations, arranged to bedirected at the same target structure.

26. A method of measuring properties of a target structure on asubstrate, a target structure including a periodic grating, the methodcomprising obtaining a spectrum from the target structure using aninspection apparatus according to any of clauses 11 to 21, andprocessing the detected spectrum so as to obtain measurements of one ormore parameters of the target structure.

27. A method according to clause 26 further comprising processing thespectrum to correct for an aberration caused by rays of the reflectedradiation being non-parallel at the location of the phase grating.

28. An optical apparatus including illumination optics for focusing abeam of radiation at a spot on a target structure, wherein theillumination optics include a filter provided at or near a pupil plane,the filter imposing on the beam a transmission loss increasing withradial distance from an optical axis of the beam.

29. An apparatus according to clause 28 wherein the filter comprises atransparent substrate having a metallic coating whose thicknessincreases radially away from a central point.

30. An apparatus according to clause 29 wherein the thickness increasesslowly at first and then at an increasing rate with increasing distancefrom the central point, such that a radial transmission function of thefilter approximates a Hanning function.

31. An apparatus according to clause 28, 29 or 30 wherein theillumination optics comprises one or more reflective focusing elements.

32. An apparatus according to clause 28, 29, 30 or 31 further comprisinga source of UV radiation to form the beam.

33. A method of measuring properties of a target structure on asubstrate, the method comprising illuminating the target structure witha spot of radiation using an optical apparatus according to any ofclauses 28 to 32, detecting radiation diffracted by the target structureand processing the detected radiation to obtain measurements of one ormore parameters of the target structure.

34. A spectroscopic scatterometer having illumination optics fordirecting broadband radiation with an angle of incidence at a spot on atarget structure, detection optics for receiving radiation diffracted ata zero or higher order from the target structure and for forming anddetecting a spectrum of the diffracted radiation, wherein the detectionoptics comprises a symmetric diffraction grating arranged to form asymmetrical pair of spectra of the reflected radiation, and wherein apair of spectrum detectors are arranged to detect both of the spectra,the apparatus further comprising a processor for comparing measurementsfrom both detectors to obtain information of an apparent surfacedepression at the location of the target structure on a substrate.

35. A method of performing a lithographic patterning step by applying apattern to a substrate using a projection system, wherein focusing ofthe projection system being based on height measurements made at aplurality of locations across the substrate, and wherein a correction isapplied to the height measurements based on information of an apparentsurface depression obtained using a spectroscopic scatterometeraccording to clause 34.

36. A method of manufacturing a device comprising applying one or moredevice patterns to a substrate by a method according to clause 35 andprocessing the patterned substrate to form devices in accordance withthe applied patterns.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic and metrology apparatus describedherein may have other applications, such as the manufacture ofintegrated optical systems, guidance and detection patterns for magneticdomain memories, flat-panel displays, liquid-crystal displays (LCDs),thin film magnetic heads, etc. The skilled artisan will appreciate that,in the context of such alternative applications, and use of the terms“wafer” or “die” herein may be considered as synonymous with the moregeneral terms “substrate” or “target portion”, respectively. Thesubstrate referred to herein may be processed, before or exposure, infor example a track (a tool that typically applies a layer of resist toa substrate and develops the exposed resist), a metrology tool and/or aninspection tool. Where applicable, the disclosure herein may be appliedto such and other substrate processing tools. Further, the substrate maybe processed more than once, for example in order to create amulti-layer IC, so that the term substrate used herein may also refer toa substrate that already contains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the present invention in the context of opticallithography, it will be appreciated that the present invention may beused in other applications, for example imprint lithography, and wherethe context allows, is not limited to optical lithography. In imprintlithography a topography in a patterning device defines the patterncreated on a substrate. The topography of the patterning device may bepressed into a layer of resist supplied to the substrate whereupon theresist is cured by applying electromagnetic radiation, heat, pressure ora combination thereof. The patterning device is moved out of the resistleaving a pattern in it after the resist is cured.

While the embodiments above have been designed to operate usingradiation at UV wavelengths, the designs can be adapted by the skilledperson to use a different or wider range of wavelengths, if desired. Theterms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.,having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g., having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the present invention have been describedabove, it will be appreciated that the present invention may bepracticed otherwise than as described. For example, the presentinvention, particularly with regard to the control of the measurementprocess and processing of the results for calibration andreconstruction, may take the form of a computer program containing oneor more sequences of machine-readable instructions describing a methodas disclosed above, or a data storage medium (e.g., semiconductormemory, magnetic or optical disk) having such a computer program storedtherein.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the present invention as described without departing fromthe scope of the claims set out below.

It is to be appreciated that the Detailed Description section, and notthe Summary and Abstract sections, is intended to be used to interpretthe claims. The Summary and Abstract sections may set forth one or morebut not all exemplary embodiments of the present invention ascontemplated by the inventor(s), and thus, are not intended to limit thepresent invention and the appended claims in any way.

The present invention has been described above with the aid offunctional building blocks illustrating the implementation of specifiedfunctions and relationships thereof. The boundaries of these functionalbuilding blocks have been arbitrarily defined herein for the convenienceof the description. Alternate boundaries can be defined so long as thespecified functions and relationships thereof are appropriatelyperformed.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the present invention that others can, byapplying knowledge within the skill of the art, readily modify and/oradapt for various applications such specific embodiments, without undueexperimentation, without departing from the general concept of thepresent invention. Therefore, such adaptations and modifications areintended to be within the meaning and range of equivalents of thedisclosed embodiments, based on the teaching and guidance presentedherein. It is to be understood that the phraseology or terminologyherein is for the purpose of description and not of limitation, suchthat the terminology or phraseology of the present specification is tobe interpreted by the skilled artisan in light of the teachings andguidance.

The breadth and scope of the present invention should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and their equivalents.

1. A inspection apparatus comprising: a spectroscopic scatterometercomprising: illumination optics configured to direct broadband radiationwith an angle of incidence at a spot on a target structure, the targetstructure in use comprising a periodic grating; zero order detectionoptics configured to receive radiation reflected from the target and forforming and detecting a spectrum of the reflected radiation; and higherorder detection optics configured to receive radiation diffracted at oneor more higher orders by the periodic grating in the target structure,and to form and detect a spectrum of the received diffracted radiation.2. The inspection apparatus of claim 1 wherein the higher orderdetection optics comprise: a focusing arrangement configured to focusthe higher order diffracted radiation emanating from the spot into animage spot comprising all wavelengths of radiation, at least one higherorder spectrum detector positioned to at least one side of the imagespot, and a diffraction grating positioned in an optical path betweenthe spot and the image spot so as to form a spectrum of the higher orderdiffracted radiation on the higher order spectrum detector.
 3. Theinspection apparatus of claim 2 wherein: the diffraction grating issymmetrical so as to form a symmetrical pair of spectra of the higherorder diffracted radiation, and a pair of spectrum detectors areprovided so as to capture both of the pair of spectra, a processorconfigured to combine measurements from both detectors to obtain asingle detected spectrum.
 4. The inspection apparatus of claim 1 whereinthe illumination optics and zero order detection optics are configuredto process radiation generally in a first plane perpendicular to a planethe target structure, while the higher order detection optics arearranged to process radiation in a second plane, angled relative to thefirst plane, the higher order diffracted radiation entering the secondplane during use of the apparatus as a result of an oblique orientationof lines the periodic grating in the target structure, relative to thefirst plane.
 5. The inspection apparatus of claim 1 arranged foroperation with radiation at wavelengths shorter than 400 nm, wherein theillumination optics comprise one or more curved mirrors for focusing theradiation into a spot on the target structure, while the zero and higherorder detection optics each comprise one or more curved mirrors forfocusing an image of the spot at a point to one side of a spectrumdetector.
 6. An inspection apparatus including a spectroscopicscatterometer, comprising: illumination optics configured to directbroadband radiation with an angle of incidence at a spot on a targetstructure; detection optics configured to receive radiation diffractedat a zero or higher order from the target structure and to form anddetect a spectrum of the diffracted radiation, wherein the detectionoptics comprises a symmetric diffraction grating arranged to form asymmetrical pair of spectra of the reflected radiation, and wherein apair of spectrum detectors are arranged to detect both of the spectra,and a processor configured to combine measurements from both detectorsto obtain a single detected spectrum of the reflected radiation.
 7. Theinspection apparatus of claim 6 wherein a further detector is arrangedto receive a zero order beam of the grating, the further detector beinglocated at a point generally in between the pair of spectrum detectors,and wherein signals from the further detector are used for monitoringfocus of the spot on the target structure.
 8. The inspection apparatusof claim 6 comprising: first and second spectroscopic scatterometers ofgenerally similar form arranged for simultaneous measurement of the sametarget structure, wherein the illumination optics and detection opticsof each scatterometer are arranged to process radiation generally in afirst plane perpendicular to a plane of the target structure, andwherein a first plane of the second scatterometer is arranged at anangle to the first plane of the first-mentioned scatterometer.
 9. TheInspection apparatus of claim 6 wherein: the detection optics isarranged to capture zero order diffracted radiation, being radiationreflected from the target structure, the detection optics include ananalyzing polarizer in a path of the reflected radiation prior to thephase grating, the analyzing polarizer comprises a Rochon prism arrangedto transmit ordinary and extraordinary rays in different directionsthereby to select only one polarization for the formation of thespectra, and the phase grating has lines oriented at an oblique anglerelative the selected polarization, whereby radiation of a differentpolarization will have its spectra formed by the phase grating at alocation away from the pair of spectrum detectors.
 10. The inspectionapparatus of claim 6 wherein the detection optics comprise: a focusingarrangement configured to focus the diffracted radiation emanating fromthe spot into an image spot at a location generally in between thespectrum detectors, the diffraction grating being positioned in anoptical path of the focusing arrangement between the spot and the imagespot so as to form the spectra of the diffracted radiation on thespectrum detectors.
 11. The inspection apparatus of claim 10 arrangedfor operation with radiation at wavelengths shorter than 400 nm, whereinthe focusing arrangement comprises one or more curved mirrors.
 12. TheInspection apparatus of claim 6 further comprising a processor forprocessing the spectrum to correct for an aberration caused by rays ofthe reflected radiation being non-parallel at the location of the phasegrating.
 13. A method comprising: measuring properties of a targetstructure on a substrate, the target structure including a periodicgrating, and obtaining a zero order spectrum and at least one higherorder spectrum from the target structure using an inspection apparatus;and processing the measured spectra so as to obtain measurements of oneor more parameters of the target structure.
 14. A method comprising:measuring properties of a target structure on a substrate, the targetstructure including a periodic grating, obtaining a spectrum from thetarget structure using an inspection apparatus; and processing thedetected spectrum so as to obtain measurements of one or more parametersof the target structure.
 15. An optical apparatus including:illumination optics configured to focus a beam of radiation at a spot ona target structure, the illumination optics include a filter provided ator near a pupil plane, the filter imposing on the beam a transmissionloss increasing with radial distance from an optical axis of the beam.16. The apparatus of claim 15 wherein the filter comprises a transparentsubstrate having a metallic coating whose thickness increases radiallyaway from a central point.
 17. The apparatus of claim 16 wherein thethickness increases slowly at first and then at an increasing rate withincreasing distance from the central point, such that a radialtransmission function of the filter approximates a Hanning function. 18.A method comprising: measuring properties of a target structure on asubstrate, illuminating the target structure with a spot of radiationusing an optical apparatus detecting radiation diffracted by the targetstructure; and processing the detected radiation to obtain measurementsof one or more parameters of the target structure.
 19. A spectroscopicscatterometer comprising: illumination optics configured to directbroadband radiation with an angle of incidence at a spot on a targetstructure, detection optics configured to receive radiation diffractedat a zero or higher order from the target structure and for forming anddetecting a spectrum of the diffracted radiation, wherein the detectionoptics comprises a symmetric diffraction grating arranged to form asymmetrical pair of spectra of the reflected radiation, and wherein apair of spectrum detectors are arranged to detect both of the spectra,and a processor configured to compare measurements from both detectorsto obtain information of an apparent surface depression at the locationof the target structure on a substrate.
 20. A method comprising:performing a lithographic patterning step by applying a pattern to asubstrate using a projection system, focusing of the projection systembeing based on height measurements made at a plurality of locationsacross the substrate, and a correction is applied to the heightmeasurements based on information of an apparent surface depressionobtained using a spectroscopic scatterometer.
 21. A method comprising:manufacturing a device by applying one or more device patterns to asubstrate; and processing the patterned substrate to form devices inaccordance with the applied patterns.